Several manufacturing processes require application of a pattern or patterns to web materials. Examples include printing, and the manufacture of electronic assemblies on flexible substrates. When multiple patterns are applied in sequence, proper alignment or registration must be achieved between patterns. Registration errors can cause misalignment between process steps or layers, as shown in FIGS. 1 and 2. In these figures, the “+” and “●” symbols represent patterns applied in different process steps. FIG. 1 shows correct registration, where all “+” and “●” symbols are aligned. FIG. 2 shows registration error resulting from positional misalignment between the two process steps.
Registration precision is limited by manufacturing process hardware, and also by the dimensional stability of the web substrate. When the web material is dimensionally unstable, then no amount of precise position control will lead to correct registration. This is shown in FIG. 3 where the web has undergone a dimensional change between process steps. As a result, most points are not correctly aligned, even though points on the left side near the web centerline are correctly positioned.
Commonly used web materials such as polyethylene terephthalate (PET) exhibit poor dimensional stability relative to requirements, for display manufacture for example. They have a high coefficient of thermal expansion, experience hygroscopic expansion in humid environments, and can exhibit anisotropic shrink when exposed to moderately high temperatures. These material properties prevent high-precision registration using position-controlled web conveyance systems, leading instead to results such as those shown in FIG. 3. Nonetheless, PET is desirable for some final products because of its transparency, light weight, flexibility, durability, and toughness. A method of precisely registering multiple patterns on a web with limited dimensional stability is needed.